Telephone systems in general use both copper wires and optical fibers to carry telephone signals. Much of the physical equipment already in place is copper wire and its associated equipment. Traditionally, a pair of copper wires was used to carry only a single conversation. Nowadays a single pair of copper wires can carry multiple conversations by way of multiplexing. Multiplexing increases the capacity of the telephone system by allowing the system to carry more calls. One type of multiplexing uses a digital carrier signal. The audio information is digitized and sent over the wires on the digital carrier. Examples of standard digital carriers that have been adopted by the telephone industry are T-1 and T-2. These are pulse code modulation carriers.
Field technicians working on a telephone cable of copper wire must frequently locate those particular copper wires in the cable bundle that have carrier signals on them. The field technician may be tracing a particular pair of wires or he may be attempting to locate an unused pair, that is a pair without a carrier signal thereon, in the cable. He can then use this unused pair to communicate with other technicians along the cable.
At voice frequencies, where a pair of copper wires carries only a single conversation, the technician can use simple test equipment to detect a pair that is in service. The test equipment may either capacitively or conductively load the wire pair. At voice frequencies (about 200-3200 Hz), this is acceptable because there is little or no degradation of service. However, at the higher frequencies used by digital carriers (772 KHz on up), such loading by test equipment can cause degradation and interruption of telephone services on the wire pair. Thus, some type of non-invasive equipment must be used when testing a pair of wires to determine the presence or absence of a digital carrier.
In the prior art of which we are aware, there is a non-invasive apparatus for detecting digital carriers. The prior art apparatus uses inductive or capacitive coupling to obtain a signal from the wire pair. The signal is filtered so as to pass only carrier frequencies. The signal is then amplified and detected. When the signal exceeds a certain threshold, it sounds a buzzer, thereby indicating the presence of a carrier.
The prior art apparatus suffers from the disadvantages of being unable to provide information on relative carrier signal strength and on carrier identity. Relative carrier signal strength is useful to determine if the carrier signal that is detected on a particular pair of wires is due to the pair actually being in service or instead is due to only a crosstalk signal that has been coupled to the pair that is being probed. Determination of carrier identity is useful in tracing lines through the local network and in particular in the central office.